Baseband direct current (DC) offset correction is commonly used in zero intermediate frequency (ZIF) and very low intermediate frequency (VLIF) receivers to eliminate offsets due to local oscillator leakage, filter flicker noise, and circuit mismatches. This correction is necessary to avoid clipping effects within the baseband analog signal path as well as to meet the desired receiver performance under weaker signal conditions (e.g., receiver sensitivity). Baseband gain control with large coarse steps can be employed in a receiver to achieve the necessary receiver dynamic range at a low cost and current drain while minimizing the amount of RF gain control range required. However, in continuous data reception (i.e., non-time division multiplexed) receivers such as that for wideband code-division multiple access (WCDMA) receivers, there needs to be a mechanism to quickly and accurately compensate for large DC offset steps which occur as a result of large baseband gain steps. If this is not achieved, receiver performance degradation can occur due to long DC and clipping transients. The large baseband gain changes can occur not only due to a larger step size of the baseband gain control stage but also due to RF Low Noise Amplifier (LNA) gain replacements within the baseband gain control stage. The RF LNA gain replacement is necessary to avoid applying large gain steps at the input to the baseband filters as well as to maintain a constant signal level into the received signal detector. In addition, RF standards such as High Speed Downlink Packet Access (HSDPA), a packet-based data service 3.5 G WCDMA standard, expect a large constant signal-to-noise ratio at the receiver analog-to-digital input where this is also necessary.
Accordingly it is desirable to provide a method and apparatus for DC offset correction which quickly and accurately compensates for large DC offset steps which occur as a result of large baseband gain steps while maintaining a constant signal-to-noise ratio at the receiver analog-to-digital input. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.